Treatment of waste and a rotary kiln therefor

ABSTRACT

Method and apparatus for treating waste materials to produce aggregates wherein pellets comprised of a mixture of solids, liquid wastes and clay are dried by hot air in a dryer. The pellets are then fed to the pyrolysis zone of a rotary kiln wherein they are heated by hot gas from an oxidation zone of the kiln to drive off most of the volatile gases. Remaining volatile gases and the fixed carbon in the waste are oxidized in the oxidation zone, and the silicates in the clay are vitrified in a vitrification zone of the kiln to produce aggregates which are crushed and screened. Volatile gases given off from the pyrolysis zone are mixed with exhaust gases from the dryer to feed the burner for the vitrification zone and to supply oxygen to the oxidation zone.

This application is a continuation of application Ser. No. 07/240,127,filed May 20, 1988, now U.S. Pat. No. 4,890,563.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a method of, and apparatus for, the treatmentof waste. In particular, the apparatus preferably includes a rotarykiln.

(2) Prior Art

The disposal of urban solid waste and/or liquid wastes is a majorproblem facing modern man. Waste materials generally have a negativeeconomic value and many toxic wastes are extremely difficult toneutralize or destroy.

Examples of waste treatment methods are disclosed in U.S. Patent Nos.1,859,159 (Greenwalt); 3,249,551 (Bixley): 3,383,228 (Rekate et al);3,957,528 (Ott et al) and 4,112,033 (Linge).

SUMMARY OF THE PRESENT INVENTION

It is an object of the present invention to provide a method for thetreatment of waste materials.

It is a preferred object to provide a method where the end product canbe used as an aggregate, where the aggregate has a positive economicvalue e.g. for concrete production, road making or as porous fill.

It is a further preferred object to provide a rotary kiln suitable forthe method.

Other preferred objects will become apparent from the followingdescription.

In one aspect the present invention resides in a method for thetreatment of waste materials including the steps of:

mixing solid and/or liquid wastes with a binding material;

passing the mixture through an extruder to form pellets of waste:

passing the pellets through a pyrolysis zone in a furnace or kiln todrive off at least portion of the volatile gases in the waste:

passing the pellets through an oxidation zone in the furnace or kilnwhere the excess oxygen causes the remaining volatile gases and at leasta portion of the fixed carbon in the waste to be oxidized; and

passing the pellets through a vitrification zone in the furnace or kilnto vitrify the silicates present in the pellets to form a solidaggregate.

The binding material will be selected on the basis that, when fired, itwill form a hard inert ceramic material and may include clay, shale orsimilar material with suitable ceramic properties.

Preferably the pellets are passed through a dryer before they are fed tothe furnace or kiln, the pellets preferably being dried by acounter-current flow of hot gas from at least one of the zones of thefurnace or kiln.

Preferably the aggregate is cooled in a cooler by a flow of air, the airheated in the cooler preferably being fed to the dryer to dry thepellets.

The exhaust gas from the dryer is preferably split into two streams, onebeing fed to an afterburner and gas cleaning system, the second to theoxidation and/or vitrification zones of the furnace or kiln.

Preferably the pellets are heated to approximately 650° C. in thepyrolysis zone by the controlled burning of some of the volatiles drivenoff from the waste and by hot gases transferred from the oxidation zone,the resulting volatile-rich gas stream being used to fuel gas in theoxidation and/or vitrification zones. Some of the fuel gas may besupplied to the afterburner.

Preferably the oxidation of the pellets, to burn off the remainingvolatiles and the fixed carbon solids, occurs in the oxidation zone ator below 1000° C., the oxidation and the combustion of some of the fuelgas from the pyrolysis zone providing the heat in the oxidation zone. Aportion of the fixed carbon may be left in the pellets to promotebloating, which reduces the specific gravity (S.G.) of the finalaggregate.

A portion of the exhaust air from the dryer may be fed to the oxidationzone by a pipe passing through the vitrification zone to supply oxygento the oxidation zone.

Preferably the vitrification of the silicates occurs at 1200° C.±200° C.in the vitrification zone, the heat in the zone being generated by thethe burning of fuel gas from the pyrolysis zone and by air from thedryer.

While it is preferred that the solids and gases in the kiln flow incounter-current direction, the gas flow in the oxidation andvitrification zones may be concurrent with the solids flow, where theexhaust gas from the dryer is fed to the oxidation zone. The hot gas isthen fed to the vitrification zone and burn with some of the fuel gasfrom the pyrolysis zone to vitrify the solids. The gas from thevitrification zone is separated into two streams, one being directed tothe pyrolysis zone to provide heat and oxygen for combustion in thatzone, the other stream being directed to the gas cleaning system.

In a second aspect the present invention resides in a rotary kiln fortreatment of waste including:

a pyrolysis zone to receive pellets of solid and/or liquid waste mixedwith a binding material where the pellets are heated to drive off atleast a portion of the volatile gases contained in the waste;

an oxidation zone where excess oxygen cause the remaining volatile gasesand at least a portion of the fixed carbon in the waste to be oxidized;and

a vitrification zone where silicates in the pellets are vitrified toform a solid aggregate.

In a third aspect the present invention resides in an apparatus for thetreatment of wastes including:

a dryer for pellets of solid and/or liquid waste mixed with a bindingmaterial:

the rotary kiln hereinbefore described: and

a cooler for the solid aggregate.

Other features of the apparatus will become apparent to the skilledaddressee.

BRIEF DESCRIPTION OF THE DRAWINGS

To enable the invention to be fully understood, a number of preferredembodiments will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic layout of a waste treatment plant;

FIG. 2 is a schematic layout of a first embodiment of the rotary kiln ofthe present invention;

FIG. 2a is a schematic layout of an afterburner, gas scrubber, andexhaust stack

FIG. 3 is a schematic layout of a second embodiment of the rotary kiln;

FIG. 3a is a schematic layout of an afterburner, gas scrubber, andexhaust stack

FIG. 4 is a schematic layout of a third embodiment of the rotary kiln;

FIG. 4a is a schematic layout of an afterburner, gas scrubber, andexhaust stack

FIG. 5 is a schematic layout of the solids and gas flow.

FIG. 6(a) is a graph of the solid and gas temperatures in the dryer andthe pyrolysis, oxidation/vitrification zones of the kiln and the cooler;and

FIG. 6(b) is a graph of the oxygen (O₂) content in the dryer and thepyrolysis, oxidation/vitrification zones of the kiln and the cooler.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, urban solid waste 10 is dumped in a receival pileand is fed as required by an apron feeder 11 to a primary pulverizer 12.The pulverized waste is transported via a conveyor 13 to a magneticseparator 14 which removes any steel components in the waste to a scrapsteel bin at 15. The waste is transported by a conveyor 16 to asecondary pulverizer 17 and is ground to size before being transportedto storage (not shown) by a conveyor 18. (The solid waste passes asecondary magnetic separator to remove any remaining metal to scrap at18a.)

Clay 19 is crushed/hammermilled and screened in a processing unit 20before being fed to a predryer/premixer 21 where it is mixed with thesolid waste transported from its store by a conveyor 22.

The dried pulverized solid waste/clay mixture is mixed in a mixer 24with liquid waste 25 from a storage tank 26 and is fed to a pugmill 27to produce pellets 28 which are conveyed to a dryer 29, to behereinafter described in more detail, by a conveyor 30. The pellets aredried and then transported to the rotary kiln 31 to be fired, ashereinafter described. The resultant aggregate is cooled and then fed toa crushing and screening plant 32 before being fed to a stockpile (notshown) by a conveyor 33.

Referring now to FIG. 2 and 2a, the pellets are conveyed by the conveyor30 to a feed hopper 34 and are supplied to the rotary dryer 29 by ascrew conveyor 35.

The dryer 29 has a drum 36 (with internal flights to convey the pelletsupward for more efficient heat transfer) mounted on a rotary supportstructure 37. A hood 38 at the exhaust end receives hot dry gas whichflows counter-current to the pellet flow to cause the latter to bedried. The hood has a discharge outlet 39 connected to a screw conveyor40 which transports the dried pellets to a surge bin 41 at the inlet endof the kiln 31. An exhaust hood 42 at the inlet end of the dryer 29receives the exhaust gas from the dryer being drawn off by a fan 43. Theoutlet of the fan is connected to an afterburner 44 and to rotary kilnsas hereinafter described. The afterburner 44 is connected to a gasscrubber 45 and exhaust stack 46.

The pellets are fed from the surge bin 41 to the inlet end of thepyrolysis zone 47 of the rotary kiln 31.

The pyrolysis zone has a rotary tube 48 lined with refractory material(and lifters to improve heat transfer efficiency) and is mounted on arotary support structure 49. A hood 50 is provided at the inlet end ofthe pyrolysis zone. The exhaust end of the zone has a reduced neckportion 51 co-axially received within the inlet end of the oxidationzone 52 of the kiln 31.

The oxidation and vitrification zones 52, 53 of the kiln 31 are formedby a second refractory-lined tube 54 mounted on a rotary supportstructure 55. The angles of inclination and the rotational speeds of thetubes 48, 54, and the neck portion 51, control the flow of the pelletsand aggregate through the kiln.

A hood 56 surrounds the inlet end of the oxidation zone 52 (and theexhaust end of the pyrolysis zone 47) to receive exhaust gases from theoxidation zone 52.

A hood 57 surrounds the discharge end of the vitrification zone 53 andleads to a discharge chute 58 for the aggregate which directs the latterto an aggregate cooler 59 and then via the cooler discharge 59a andconveyor 59b to the crushing and screening plant 32.

The respective solids and gas flows will now be described with referenceto FIG. 5.

A. Solids Flow

From the pugmill 27 the extruded pellets 28 enter the rotary drum dryer29 (via screw conveyor 35) where most of the water in the pellets isremoved. The water is evaporated into a hot air stream travellingcounter-currently to the pellets. This hot air is generated by heatingfresh air 60 by direct contact with hot aggregate product in theaggregate cooler 59, and its flow and temperature are regulated toachieve the required degree and rate of drying.

The pellets 28 pass from the dryer 29 by way of screw conveyor 40 intothe surge bin 41 which acts as a buffer between the dryer 29 and thepyrolysis zone 47. Pellets are removed as required from the bin 41 intothe pyrolysis zone. Here, carefully controlled burning of some of thevolatiles in the pellets and heat transfer from the oxidation zone hotgas raises the solids temperature to approximately 650° C., enough todrive off most of the volatiles present. The resulting volatile-rich gasstream is drawn off via the hood 50 and is used as fuel gas elsewhere inthe process. Any moisture remaining in the pellets is also driven off atthis stage.

From the pyrolysis zone 47 the pellets pass into the oxidation zone 52.The pellets first enter the oxidation zone where they are contactedagainst excess oxygen, and the remaining volatiles and fixed carbon areoxidized. This oxidation along with combustion of some of the fuel gasfrom the pyrolysis zone 47 provides the heat to raise the solidstemperature to approximately 1100° C.

The pellets then pass into the vitrification zone 53 of the kiln.Further fuel gas from the pyrolysis zone 47 is burnt here to bring thesolids temperature up to 1200° C.±200° C. This final heating stagebrings about vitrification of silicates present in the pellets,conferring desirable physical properties on the aggregate produced.

The hot aggregate passes from the kiln directly into the verticalaggregate cooler 59 where it is cooled against a counter-current flow offresh air 60.

B. Gas Flow

Fresh air 60 enters the aggregate cooler 59, where it absorbs heat fromthe hot aggregate. This hot air stream is introduced to the raw feeddryer (via hood 38), counter-current to the flow of pellets. In thedryer 29 this gas stream evaporates most of the water from the pellets,cooling as it does so. The flow and temperature of the gas is set toachieve optimum drying conditions.

The exhaust gas from the dryer is controlled by hood 42 and is splitinto two streams. A large portion goes directly into the afterburner 44for burning prior to gas cleaning.

Gas and solids flow in a counter-current manner in theoxidation/vitrification zones of the kiln. A proportion of fuel gasgenerated in the pyrolysis section is fed to a burner 61 located in thevitrification zone 33 of the kiln and the remaining air from the dryer29 is introduced into the oxidation and vitrification zones. Some ofthis air is fed to the fuel gas burner 61 in the vitrification zonewhere gas temperatures are increased and some of the heat given upraising the solid temperatures to that required for vitrification. Theremaining air introduced to the oxidation/vitrification zones is fed tothe oxidation zone 52 through a pipe 62 running down the centre of thevitrification zone 53. The air is pre-heated in this pipe 62 and ondelivery to the oxidation zone 52 supplies the oxygen required forcombustion of the remaining volatiles and fixed carbon in the pyrolysedpellets thereby releasing sufficient heat to raise the solids from thepyrolysis zone temperature to that required for oxidation. Hot gasleaving the oxidation zone 52 is in part returned to the pyrolysis zone47 via the neck portion 51 counter-current to the pellet flow from thepyrolysis zone 47 to the oxidation zone 52 to provide heat and oxygenfor combustion in that section of the process.

This stream is controlled to allow a small fraction of the volatilespresent in the pellets to be burnt. The heat released by combustion andheat transfer from the hot oxidation zone gas raises the solidstemperature sufficiently to drive off most of the volatiles present intothe gas stream, in the pyrolysis zone. The resulting oxygen free,volatile-rich gas stream from this pyrolysis zone 47 is discharged fromhood 50 and is used as fuel gas in the vitrification zone 53 and theafterburner 44.

The remainder of the off gas from the oxidation zone (i.e. not going topyrolysis) passes through the hood 56 directly to the gas scrubber 45.

C. Alternative Gas Flow (as shown in dashed lines in FIG. 5)

As an alternative to the above, the gas flow in theoxidation/vitrification zones 52/53 can be co-current instead ofcounter-current to the solids flow. In this case the air returning tothe process from the dryer 29 is introduced into the oxidation zone 52of the kiln. Here it allows the combustion of remaining volatiles andthe fixed carbon in the pyrolysed pellets and absorbs that heatgenerated that is not taken up by the solids. The hot gas then passesinto the vitrification zone 53 where its temperature is furtherincreased by burning a portion of the fuel gas from the pyrolysis zone47. Some of this heat is given up to bring the solids up tovitrification temperature. Gas leaving the vitrification zone 53 is inpart returned to the pyrolysis zone 47 to provide heat and oxygen forcombustion in that section of the process. The remainder of the gas isdischarged from the plant via hood 57 to the gas scrubber 45.

The temperature of the waste and of the gases in the dryer 29, pyrolysiszone 47, oxidation/vitrification zones 52, 53 and the cooler 59 areshown in solid and dashed lines respectively in FIG. 6(a) while theoxygen (O₂) content in the above is shown in FIG. 6(b).

Referring now to FIG. 3 and 3a, the layout of the rotary kiln isgenerally the same as for FIG. 2 and 2a, except that the pyrolyzedpellets are transported from the exhaust end of the pyrolysis zone 47 tothe inlet end of the oxidation zone 52 by a tube 70 passing through thehood 56 and where exhaust gas from the oxidation zone is fed to hood 71at the exhaust end of the pyrolysis zone via an inlet pipe 72.

In the embodiment of FIG. 4 and 4a, the oxidation and vitrificationzones 52, 53 are provided in respective refractory-lined tubes 54a, 54bmounted on respective rotary supports 55a, 55b. The oxidized pellets areconveyed from the exhaust end of the oxidation zone 52 to the inlet endof the vitrification zone 53 via a hood 73 and discharge chute 74, thelatter being received in a hood 75 in the vitrification zone.

The hot gases from the vitrification zone 53, through duct 43, are mixedwith some of the exhaust gases from the dryer 29 and fed into theoxidation zone 52, thereby eliminating pipe 62 through the vitrificationzone 53.

As will be readily apparent to the skilled addressee, the processvariations provide the potential customer with a range of options fromwhich to choose.

The aggregate can be used for concrete production, road making or highgrade porous fill.

Variations and modifications may be made to the embodiments describedwithout departing from the present invention as defined in the appendedclaims.

We claim:
 1. A method for the treatment of waste materials including thesteps of:mixing solid and/or liquid wastes with a binding material;forming pellets of waste material from said mixture; passing the pelletsthrough a pyrolysis zone in a kiln to drive off at least a portion ofthe volatile gases in the waste; passing the pellets through anoxidation zone in a kiln where the oxygen causes the remaining volatilegases and at least a portion of the fixed carbon in the waste to beoxidized; passing the pellets through a vitrification zone in a kiln tovitrify the silicates present in the pellets to form a solid aggregate,and wherein the volatile gases from the pyrolysis zone are used as fuelgas for combustion in the oxidation and/or vitrification zones.
 2. Amethod according to claim 1, wherein:the pellets are passed through adryer upstream of a kiln, the pellets being dried by a counter-currentflow of hot gas from at least one of the zones of one of said kilns. 3.A method according to claim 2 wherein:the aggregate is cooled in acooler by a flow of air, the heated air exhausted from the aggregatecooler being fed to said dryer in a counter-current flow to the pellets.4. A method according to claim 1 wherein:the binding material is clay,shale or similar material which, when fired, forms a hard ceramicmaterial.
 5. A method according to claim 1, wherein:the flow of thepellets and gases in the pyrolysis zone are counter-current, and theflow of the pellets and the gases in the oxidation and vitrificationzones are counter-current or concurrent.
 6. The method according toclaim 1 wherein:said pellets are heated in said pyrolysis zone toapproximately 650° C., are heated in said oxidation zone toapproximately 1,000° C., and are vitrified in said vitrification zone atapproximately 1,200° C.±200° C.
 7. A kiln system for treatment of wastematerials comprising:means defining a pyrolysis zone adapted to receivepellets of solid and/or liquid waste mixed with a binding material, andmeans for heating said pellets in said zone to drive off at least aportion of the volatile gases contained in the waste; means defining anoxidation zone downstream of said pyrolysis zone in which an excessoxygen environment causes the remaining volatile gases and at least aportion of the fixed carbon in the waste to be oxidized; means defininga vitrification zone downstream of said oxidation zone in whichsilicates in the pellets are vitrified to form a solid aggregate, andmeans for delivering said volatile gases formed in said pyrolysis zoneto said oxidation and vitrification zones to be used as fuel gastherein, said fuel gas supplying the fuel for the process.
 8. Theapparatus according to claim 7 wherein:said means defining saidpyrolysis zone comprises a first refractory lined tube; said meansdefining said oxidation and vitrification zones comprises a secondrefractory lined tube; and wherein said first tube is formed with a neckportion which controls both the flow of pellets from the pyrolysis zoneto the oxidation zone and the counter-current flow of hot gas from theoxidation zone to the pyrolysis zone to provide heat and oxygen forcombustion in said pyrolysis zone.
 9. The apparatus of claim 8 furtherincluding a dryer upstream of said pyrolysis zone, means for dryingpellets conveyed to said dryer, and a cooler downstream of saidvitrification zone for cooling the solid aggregate.
 10. Apparatusaccording to claim 9 wherein:hot gas from at least one of the zones ofthe kiln and/or hot air from the cooler is fed to the dryer incounter-current flow to the flow of pellets in the dryer.
 11. Theapparatus of claim 7 wherein said means for heating said pyrolysis zonecomprises hot gases delivered to said pyrolysis zone from oxidationzone.
 12. The apparatus of claim 10 further including:means fordelivering volatile fuel gas from said pyrolysis zone to saidvitrification zone for combustion therein, and means for delivering aportion of the exhaust gases from the dryer for combustion in saidvitrification zone.
 13. The apparatus of claim 8 wherein said first andsecond tubes are inclined downwardly for gravity feed of said pellets,and are independently rotated about their longitudinal axes.
 14. Theapparatus of claim 7 wherein said means for delivering said volatilegases from said pyrolysis zone to said oxidation and vitrification zonescomprises a first burner located adjacent the discharge end of saidvitrification zone, and a second burner extending through saidvitrification zone into said oxidation zone.